为什么这个 Ajax 需要这么长时间才能渲染

Why is this Ajax taking so long to render?

本文关键字：长时间 Ajax 为什么      更新时间：2023-09-26

我有一个页面，主要包含Ajax生成的动态内容。它每 30 秒随机化一次，并显示数据库中的新内容。PHP似乎很好，但是我的Javascript代码似乎有问题，或者数据库存在问题导致其滞后（Ajax加载大约需要30秒！

似乎在我的 Javascript 中对 setInterval 的递归调用在执行函数之前等待分配的毫秒数，但我在我的 Javascript 中找不到任何错误.

此外，从数据库中检索到两个图像 url 字符串，当 Ajax 必须从外部来源检索信息时，它似乎可能会滞后.

或者我的使用是否有滞后的 PHP MySQL语法ORDER BY rand() ？

这是相关的 html：

    <html>
    <head>
    <title></title>
    <script type = "text/javascript" src = "randomProducts.js" />
    <script type = "text/javascript">
    setIntervalOnload();
    </script>
    </head>
    <body>
    ...
    ...
    ...
    </body>
    </html>

以下是相关的 Javascript：

    // global static variables
        var subCategory; // initialized from setCategoryTree
        var t; // for setInterval and clearInterval
        var seconds;
        var millisecondsPerSecond;
        var milliseconds;
    function setIntervalOnload()
    {
        getRandomProducts();
        if(typeof t != "undefined")
            clearInterval(t);
        seconds = 30;
        millisecondsPerSecond = 1000;
        milliseconds = seconds * millisecondsPerSecond;
        t = setInterval(getRandomProducts, milliseconds);
    }
    function getRandomProducts()
    {
        //window.alert(subCategory);
        if(typeof subCategory == "undefined")
            subCategory = "all";
        else
        {
            clearInterval(t);
            t = setInterval(getRandomProducts, milliseconds);
        }
        var req = new XMLHttpRequest();
        var products = document.getElementById("products");
        req.onreadystatechange = function()
        {
            if( (req.readyState == 4) && (req.status == 200) )
            {
                var result = req.responseText;
                products.innerHTML = result;
            }
        };
        req.open("GET", "randomProducts.php?category=" + subCategory, true);
        req.send(null);
    }
    function setCategoryTree(link)
    {
        var categoryTree = document.getElementById("categoryTree");
        /* climbing the DOM-tree to get the category name (innerHTML of highest "a" tag) */
        var category = link.parentNode.parentNode.parentNode.getElementsByTagName("a")[0].innerHTML;
        subCategory = link.innerHTML;
        categoryTree.innerHTML = "==>&nbsp;" + category + "&nbsp;&nbsp;==>&nbsp;" + subCategory;
        getRandomProducts();
    }

。这是相关的 PHP：

<?php
    // connect to MySQL
    $dbName = "blah";
    $db = mysql_connect("localhost", $dbName, "asdf");
        if (!$db)
        {
             echo "<p>Error - Could not connect to MySQL</p>";
             exit;
        }
    // select the blah database
    $blah = mysql_select_db("blah");
        if(!$blah)
        {
            echo "<p>Error  - Could not select the blah database.</p>";
            exit;
        }
    // fix html characters in $subCategory
    $subCategory = $_GET["category"];
    trim($subCategory);
    $subCategory = stripslashes($subCategory);
    $subCategoryFixed = htmlspecialchars($subCategory);
    // for loop for each random product (total of 10 random products)
    for($i = 0; $i < 10; $i++)
    {
        // query the blah database for all products
        if($subCategoryFixed == "all")
        {
            $query = "SELECT * FROM products ORDER BY rand();";
            $result = mysql_query($query);
        }
        else // query the blah database for products in selected subCategory
        {
            $query = "SELECT * FROM products WHERE cat = " . $subCategoryFixed . " ORDER BY rand();";
            $result = mysql_query($query);
        }
            if (!$result)
            {
                echo "<p>Error - the query could not be executed</p><br />";
                $error = mysql_error();
                echo "<p>" . $error . "</p>";
                exit;
            }
        $row = mysql_fetch_array($result);
        $productValues = array_values($row);
        $name = htmlspecialchars($productValues[3]);
        $price = htmlspecialchars($productValues[5]);
        $img = htmlspecialchars($productValues[7]);
        // product info is formatted for home.html here
        $str = '<div>
                    <a href = "' . $link . '" title = "' . $name . '">
                        <table id = "product-table" onmouseover = "darkenProduct(this);" onmouseout = "lightenProduct(this);">
                            <tr>
                                <td>' . $name .'</td>
                            </tr>
                            <tr>
                                <td><img src = "' . $img . '" /></td>
                            </tr>
                            <tr>
                                <td>' . $price . '</td>
                            </tr>
                        </table>
                    </a>
                </div>';
        echo $str;
    } // end of products for loop
?>

您没有在 onload 方法中运行代码。AJAX 安装代码的运行速度可能快于页面的加载速度，因此var products = ...为 null。 您需要执行以下操作：

<body onload='setIntervalOnload();'>

或

 window.onload = setIntervalOnload;
setInterval不会

立即调用该函数。无论间隔有多长，函数的第一次运行都将延迟。

如果您希望函数立即运行以及在计时器上运行，则必须专门调用该函数。

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